A fixing device of a heat roller fixing type has been widely used in an image forming apparatus, such as a copier and a printer, employing an electrophotographic printing method. The fixing device of the heat roller fixing type generally includes a pair of rollers pressured against each other (a fixing roller and a pressure roller), and fixes a non-fixed toner image with heat and pressure by (i) heating the pair of rollers with heating means, and then (ii) feeding recording paper, on which the non-fixed toner image is formed, into a pressure area (fixing nip area) of the pair of rollers so that the recording paper passes through the pressure area. The heating means is provided inside both or one of the pair of the rollers and includes a halogen heater or the like.
Meanwhile, the fixing device provided in a color image forming apparatus generally employs an elastic roller, which is a fixing roller whose surface is covered with an elastic layer made of silicon rubber or the like. If the fixing roller is the elastic roller, then the surface of the fixing roller is elastically deformed according to an irregular surface level of the non-fixed toner image and makes contact with the non-fixed toner image with the deformed shape so that the non-fixed toner image is covered with the fixing roller that is 3-dimensionally in contact with the non-fixed toner image. This makes it possible to appropriately fix a multicolor non-fixed toner image, which holds more toner quantitatively than a black non-fixed toner image. Further, since the elastic layer deformed tries to recover its original shape in the fixing nip area, it is possible to improve releasability of the toner from the fixing roller, particularly a color toner which is more likely to offset than a black toner. Moreover, since the elastic layer of the fixing roller is deformed concavely, the fixing nip area is to have a convexed nip shape which bows upward (toward the fixing roller) in a convex curve (i.e., inverse nip shape). The inverse nip improves strippability of the recording paper from the fixing roller, thereby making the recording paper strippable without providing stripping means such as a stripping blade (i.e., self stripping is achieved). As such, it is possible to prevent an image defect which attributes to the stripping means.
However, in a case where such a fixing device provided in the color image forming apparatus operates at a high processing speed (processing speed is a speed at which the recording paper passes through the fixing nip area; i.e., fixing speed), the non-fixed toner image would not be given sufficient heat, and the non-fixed toner would not be appropriately fixed. Therefore, the fixing device cannot operate at the high processing speed without a wider nip width in the fixing nip area. Such a wider nip width will give sufficient heat to the non-fixed toner image and make it possible to appropriately fix the non-fixed toner even with the high processing speed. The nip width may be widened by (1) thickening the elastic layer of the fixing roller or (2) increasing a diameter of the fixing roller.
However, the conventional fixing roller, which includes the heating means inside thereof and whose surface is covered with the elastic layer, has the following problem. Since the elastic layer is very poor in heat conductivity, a temperature of the surface of the fixing roller does not quickly follow a temperature change of the heating means when the conventional fixing roller operates at the high processing speed. Therefore, if the elastic layer is thickened as in the configuration (1), then the temperature of the surface of the fixing roller follows more slowly the temperature change of the heating means. Further, the configuration (1) takes a long time to warm-up the fixing roller for heating, greater power consumption, because of the poor heat conductivity of the elastic layer.
Further, if the diameter of the fixing roller is increased as in the configuration (2), then a size of the fixing roller itself increases, thereby increasing a heat capacity of the fixing roller. As such, the configuration (2) takes a long time to warm up the fixing roller for the heating, and greater power consumption.
In order to solve such problems, there has been proposed fixing process employing an external heating technique, in which an external heat means is abutted against the surface of the fixing roller so as to heat the fixing roller from outside.
For example, Patent Literature 1 discloses a fixing device employing fixing process using an external belt heating technique. The fixing device includes a fixing member; an endless belt (external heat belt) suspended between a plurality of suspension rollers; and heating means for heating the endless belt. The endless belt is pressured against the fixing member so as to heat up the fixing member.
In the fixing device employing the fixing process using the external belt heating technique, the fixing roller is heated from outsize by a belt whose heat capacity is small. Therefore, the fixing roller is quickly heated, thereby shortening a time taken for warming up the fixing roller. Accordingly, it is possible to achieve a wide nip width, by providing the elastic layer of low hardness on the surface of the fixing roller or by increasing the diameter of the fixing roller while solving the problem of the temperature change of the fixing roller and the problem of the time taken for warming up the fixing roller.
However, in the technique disclosed in Patent Literature 1, a cleaning web is abutted against the surface of the fixing roller in a position upstream of the abutment of the external heat belt in a direction in which the fixing roller rotates. The cleaning web provided as such removes the toner, paper powder, or the like, which are adhered to the surface of the fixing roller, and protects the external heat belt against the toner, the paper powder, or the like. In this arrangement, since the cleaning web and the surface of the fixing roller are rubbed with each other, the rubbing would cause a scratch on the surface of the fixing roller. This scratch will lead to an image defect.
In order to solve this problem, it is an option to abut a cleaning member made of a scraper or a pad etc. against the external heat belt without providing the cleaning web which rubs with the fixing roller, thereby protecting the external heat belt and the fixing roller against the toner, the paper powder, or the like (this arrangement is not a conventional art, but the one developed by the inventors of the present invention).
Citation List
Patent Literature 1
Japanese Patent Application Publication, Tokukai, No. 2007-212896 A (Publication Date: Aug. 23, 2007)